Fixed checkbox value in table without pkey

When a table is displaying a record set without entity's pkey attribute. A checkbox value isn't properly prepared. This patch adds the preparation (converts value to string).

https://fedorahosted.org/freeipa/ticket/2404

install/ui/widget.js

@@ -1615,6 +1615,7 @@ IPA.table_widget = function (spec) {
 
         if (!select_set) {
             value = record[that.value_attr_name];
+            value = value ? value.toString() : '';
             $('input[name="'+that.name+'"]', tr).val(value);
         }